The present invention relates to an ultrasound diagnostic apparatus having a function of determining a sound velocity inside an inspection object, a method of determining a sound velocity inside an inspection object, and a recording medium in which a program determining a sound velocity of an inspection object in an ultrasound diagnosis is recorded.
Conventionally, ultrasound diagnostic apparatuses such as ultrasound image diagnostic apparatuses using ultrasound images are put to practical use in the medical field.
Generally, this type of ultrasound diagnostic apparatus has an ultrasound probe (hereinafter, referred to as “probe”) with a plurality of built-in elements (ultrasound transducers) and an apparatus main body connected with the probe. In the ultrasound diagnostic apparatus, an ultrasonic beam is transmitted toward a subject (an inspection object) so as to form a predetermined focus point (transmission focus) from the plurality of elements of the probe, an ultrasonic echo from the subject is received in the probe, and an ultrasound image is generated by electrically processing the reception signal of the received ultrasonic echo in the apparatus main body.
Here, in the ultrasound diagnostic apparatus, an ultrasonic echo from a subject according to a single transmission of the ultrasonic beam is received by a plurality of elements. Accordingly, even with ultrasonic echoes reflected by the same reflector, the reception time of the ultrasonic echoes is delayed according to the position of each of the elements.
Therefore, in the ultrasound diagnostic apparatus, an ultrasound image is generated by analog-to-digital (A/D) converting the reception signal of the ultrasonic echoes received in each of the elements into a digital reception signal (hereinafter, referred to as element data), and then carrying out a reception focusing process on the element data. That is, in the ultrasound diagnostic apparatus, reception data (sound ray signals) is generated by performing delay correction on the element data according to a delay time, matching the phases, and performing phasing addition, and an ultrasound image is generated using the reception data.
In conventional ultrasound diagnostic apparatuses, the sound velocity of ultrasonic waves in the subject is assumed to be constant, and the reception focusing process was performed by fixing a sound velocity of the ultrasonic waves to a predetermined certain value.
However, the sound velocity varies depending on the type of tissues such as fatty layers, muscular layers in a living body, and therefore the sound velocity of ultrasonic waves is not uniform in the subject. In addition, the thicknesses of fatty layers and muscular layers are different in fat subject and slim subject. In other words, the sound velocity of ultrasonic waves varies from person to person.
Accordingly, in a conventional ultrasound diagnostic apparatus in which the sound velocity of ultrasonic waves is fixed, when the actual sound velocity in a subject differs from a set sound velocity, the arrival time for the ultrasonic echoes to be reflected inside the subject and reach the elements does not match with a set delay time.
As a result, there is a problem in that proper phase matching is not possible, reception focusing is not properly performed, and the image quality of the obtained ultrasound image deteriorates. In addition, there is also a problem in that the obtained ultrasound image is distorted with respect to the actual subject.
With respect to such problems, in the ultrasound diagnostic apparatus, the sound velocity in the subject is determined (calculated), and the reception focusing process is performed using this sound velocity.
For example, JP 2011-92686 A describes an ultrasound diagnostic apparatus which transmission and reception of ultrasonic waves after setting a region of concern where a diagnosis region in a subject is divided in an ultrasound image to be photographed, calculates a focus index for each of a plurality of sound velocities (set sound velocities) set in advance in each region of concern by performing a reception focusing process with respect to obtained element data using a plurality of sound velocities (set sound velocities) set as appropriate, and uses the calculated focus indexes to determine the sound velocity (the ambient sound velocity) in each region of concern.
Examples of the focus indexes include contrast, brightness, and the like. For example, a set sound velocity where the brightness set as the focus index was the highest may be determined as the sound velocity in the region of concern.